THE EFFECT OF ANTI-DIURETIC HORMONE ON THE ENDOLYMPHATIC SAC OF THE INNER-EAR

The anti-diuretic hormone vasopressin (AVP) regulates water excretion from the kidney by increasing the water permeability of the collecting duct. AVP binds to V-2-receptors and induces the translocation of aqu aporin-2 water channels (AQP-2) into the apical plasma membrane of pri ncipal cells. By this mechanism AVP controls water reabsorption in the kidney. The effects of AVP on the endolymphatic sac (ES) of the inner ear, which is thought to mediate reabsorption of endolymph, were inve stigated. Both the V-2-receptor and the AQP-2 water channel were found to be expressed in the ES epithelium. In the ES AVP binds to receptor s most probably of the V-2-subtype. Application of AVP to organotypica lly cultured ES inhibits membrane turnover in ribosomal-rich cells of the ES epithelia, which is thought to mediate translocation of AQD-2 i nto the surface membrane. This suggests that AVP has contrasting effec ts in the inner ear and kidney, which may be physiologically useful fo r maintaining endolymphatic pressure during severe hypovolemia. Animal experiments show that AVP causes endolymphatic hydrops after systemic application to guineapigs, which suggests a causal role for the incre ased AVP levels found in humans suffering from Meniere's disease.